1. Field of the Invention
The present invention relates to a method of determining highly accurately the color tone of a coating which contains glitter.
2. Description of the Related Art
It is of great importance to determine the color tone of a coating such as a coating on an automobile body or the like for the purpose of obtaining a coated product with a desired appearance. Heretofore, the color tone of a coated surface has been determined by a skilled worker who visually compares the color tone of the coated surface and the color tones of reference patches.
However, since such a visual checking process needs a high level of skill on the part of the inspector, it has recently been proposed to measure color tones according to spectrocolorimetry. While the spectrocolorimetric process is capable of measuring the color tones of solid paint coatings, it fails to make accurate measurements of the color tones of coated surfaces produced of paints containing luster pigments known as flitter, such as metallic paints containing flakes or fragments of aluminum or the like.
FIGS. 3A and 3B of the accompanying drawings show metallic coatings on workpieces. Each of the illustrated metallic coatings comprises an inner coating layer 2, a base coating layer 4 disposed on the inner coating layer 2, and a clear coating layer 6 disposed on the base coating layer 4. The base coating layer 4 contains a luster pigment or flitter A composed of fine metal fragments or flakes such as flakes of aluminum, for example. When light reflected from the metallic coating is measured, the measured value varies depending on the orientation of the flitter A, an effect known as "flip-flop".
Japanese laid-open patent publication No. 61-61042, for example, discloses a method of measuring the color tone of a coated surface produced of a paint containing flitter. According to the disclosed method, illuminating light is applied to the coated surface, and light reflected by the coated surface is measured at angles of 15.degree., 45.degree., and 110.degree. from the direction in which the light is reflected according to regular reflection (the direction of regular reflection). A flop value FLOP is then determined from the values of lightness of the light reflected at the angles of 15.degree., 45.degree., and 110.degree. according to the following equation: ##EQU1## where k, a, b are predetermined coefficients. Using the flop value FLOP, it is possible to determine, with high accuracy, the characteristics of the color tone that varies depending on the direction in which it is observed.
The disclosed method can produce measured results that are satisfactory to some extent for paints such as solid paints, metallic paints, and pearlescent paints. However, it fails to provide a high degree of measuring accuracy with respect to interference mica colors, low-lightness colors, and colors of paints containing new glitter.